Reactivity of blastfurnace slags
The use of blastfurnace cements has proved its worth in the building industry for many decades. As they generate low heat of hydration during the hydration process, their utilisation in the manufacture of bulky concrete elements is advantageous. Their strength development is, however, slower than that of Portland cement, especially in the early hydration phases. Understanding of the chemical reactivity of blastfurnace slags and of its impact on the physical properties of corresponding cement-bound building materials is indispensable for the purposeful and efficient utilisation of cements containing blastfurnace slag. Among other factors, the role of the blastfurnace slag particles, which are subject to compressive load in the hardened cement paste matrix, and the effect of the chemical composition are of fundamental importance in this context.
Previously, researchers almost exclusively proceeded from the assumption that the strength development of building materials containing blastfurnace slag and the chemical reactivity of the blastfurnace slags contained were proportional to each other. Current investigations by the Research Institute, however, suggest the very opposite (Fig. 1). |
Correlation between hydration and technical properties
The results of physical cement testing, especially of compressive strength testing according to EN 196-1, serve as a criterion for the technical performance features of cement. The way in which the elements of the mortar structure that are formed during the hydration of the cement constituents react to pressure application is decisive for the course of strength development, which is crucial under technical aspects. According to the findings made by the Institute, the chemical conversion rate of the blastfurnace slag (“reactivity”) does not correlate with load bearing capacity (compressive strength).
Blastfurnace slag in the hardened cement paste microstructure
There are indications that it is not chemically reactive, but chemically rather inert blastfurnace slags that, together with the other blastfurnace cement constituents, turn hydration products into stable microstructure elements capable of bearing compressive loads. Chemically inert blastfurnace slags produce a microstructure that can absorb compressive loads as they possess a closely meshing, stable contact zone between blastfurnace slag glass not yet converted inside the particle and hydration products from constituents of blastfurnace slag, clinker and sulphate agent. In combination with the aggregates enclosed in the mortar and concrete microstructure, which are characterised by high compressive capacity, this results in high compressive strength in standard mortar testing. By contrast, faster reaction of blastfurnace slag with water generates distinct reaction rims around the blastfurnace slag particles. These gel-like rims have a high water content (Fig. 2). Such microstructure elements can evade pressure applied by deformation or shear motion along the boundaries of layers within the rims. When the pressure applied during compressive strength testing is not only borne by the aggregate particles, but also diverted via the blastfurnace slag particles possessing varying compressive capacity, the specimen suffers damage after a certain time. |
The role of titanium oxide
The titanium oxide content of blastfurnace slags has crucial influence on the early strength of blastfurnace cement-bound building materials, although the TiO2 proportion merely totals about 1%. A slight increase in the titanium oxide content by a mere 0.5% in absolute terms may result in a drastic decline in the early strength of blastfurnace cement. By contrast, the chemical reactivity of blastfurnace slag tends to decrease as the titanium oxide content increases. For that reason, blastfurnace slags with high titanium oxide content frequently constitute an exception to the observation that chemically reactive blastfurnace slags produce rather low compressive strengths in blastfurnace cements. It is certain, however, that a high titanium oxide content is not the only cause of low early strength (Fig. 3). |
Literature
Wassing, W.: Chemische Reaktivität und Normdruckfestigkeit. 15. Internationale Baustofftagung, 24.-27. September 2003 Weimar, Tagungsbericht Band 1, ISBN 3-00-010932-3 |
Wassing, W.: Relationship between the chemical reactivity of granulated blastfurnace slags and the mortar standard compressive strength of the blastfurnace cements produced from them. Cement International 1 (2003); Nr. 5, S. 94-104 |
Wassing, W.: Reactions of aluminium. Cement International 1 (2003), Nr. 4, S. 49 |
